The invention relates to an optical MEM device with movable ribbons for modulating light. More particularly, the present invention relates to an optical MEM device encapsulated within a dampening gas environment to reduce vibrations of the movable ribbons during operation.
Optical MEM (micro-electro-mechanical) device have applications in display, print, optical and electrical technologies. One type of an optical MEM device is a grating light valve that is capable of modulating light by constructive and destructive interference of an incident light source. Exemplary grating light valves and methods for making grating light valves are disclosed in the U.S. Pat. Nos. 5,311,360, 5,841,579 and 5,808,797, issued to Bloom et al., the contents of which are hereby incorporated by reference.
Grating light valves of the instant invention generate the condition for constructive and destructive interference through a plurality of movable ribbons. The movable ribbons provide a first set of reflective surfaces that are movable relative to a second set of reflective surfaces. The second set of reflective surfaces are reflective surfaces on a substrate element or on a second set of ribbons. In operation, an incident light source having a wavelength xcex impinges on the first set of reflective surfaces and the second set of reflective surfaces. The movable ribbons are displaced towards or away from the second set of reflective surfaces by xcex/4, or a multiple thereof. The portion of light that is reflected from the first set of reflective surfaces and the portion of light that is reflected from the second set of reflective surfaces alternate between being in phase and being out of phase. Preferably, the first set of reflective surfaces and the second set of reflective surfaces are either in the same reflective plane or are separated xcex/2 for generating the condition for constructive interference.
FIG. 1a illustrates a grating light valve with plurality of movable ribbons 100 that are formed in a spatial relationship over a substrate 102. Both the ribbons 100 and the regions of the substrate between the ribbons have reflective surfaces 104 and 106. The reflective surfaces 104 and 106 are provided by coating the ribbons 100 and the substrate with a reflective material, such as an aluminum or silver. The height difference 103 between the reflective surfaces 104 and 106 on the ribbons 100 and the substrate 102 is nxcex/2 (where n is a whole number). When light having a wavelength xcex impinges on the complement of reflective surfaces 104 and 106, the portion of light reflected from the surfaces 104 of the ribbons 100 will be in phase with the portion of light reflected from the surfaces 106 of the substrate 102. This is because the portion of light which strikes the surfaces 104 of the substrate 102 will travel a distance xcex/2 farther than the portion of light striking the surfaces 104 of the ribbons 100. Returning, the portion of light that is reflected from the surfaces 104 of the substrate 102 will travel an additional distance xcex/2 farther than the portion of light striking the surface 106 of the ribbons 100, thus allowing the complement of reflective surfaces 104 to act as a mirror.
Referring to FIG. 1b, in operation the ribbons 100 are displaced toward the substrate 102 by a distance 105 that is equal to xcex/4 or xcex/4 plus nxcex/2 (where n is a whole number) in order to switch from the conditions for constructive interference to the conditions for destructive interference. When light having a wavelength xcex impinges on the reflective surfaces 104xe2x80x2 and 106 with the ribbons 100xe2x80x2 in the down position, the portion of light reflected from the surfaces 104xe2x80x2 will be out of phase, or partially out of phase, with the portion of light reflected from the surfaces 106 and some or all of the light will be diffracted. By alternating the ribbon between the positions shown in FIG. 1a and FIG. 1b, the light is modulated.
An alternative construction for a grating light valve is illustrated in the FIGS. 2a-b. Referring to FIG. 2a, the grating light valve has a plurality of ribbons 206 and 207 that are suspended by a distance 203 over a substrate element 202. The ribbons 206 and 207 are provided with reflective surfaces 204 and 205, respectively. The surface 208 of the substrate 202 may also be reflective. The first set of ribbons 206 and the second set of ribbons 207 are initially in the same reflective plane in the absence of an applied force. Preferably, the first set of ribbons 206 and the second set of ribbons 207 are suspended over the substrate by a distance 203 such that the distances 209 between the reflective surfaces 205 and 205 of the ribbons 206 and 207 and the reflective surface 208 of the substrate 202 corresponding to n xcex/2. Accordingly, the portions of light reflected from the surfaces 204 and 205 of the ribbons 206 and 207 and the reflective surface 208 of the substrate 202 with a wavelength xcex will all be in phase. The ribbons 206 and 207 are capable of being displaced relative to each other by a distance corresponding to a multiple of xcex/4 and thus switching between the conditions for constructive and destructive interference with an incident light source having a wavelength xcex.
In the FIG. 2b, the second set of ribbons 207 is displaced by a distance 203, corresponding to a multiple of xcex/4 of to the position 207xe2x80x2. The portion of the light reflected from the surfaces 205xe2x80x2 of the ribbons 207 will destructively interfere with the portion of the light reflected from the surfaces 204 of the ribbons 206. While the FIG. 1b and FIG. 2b show ribbons touching the surface of the substrate, the instant invention is particularly useful in grating light valve designs where movable ribbons do not contact the substrate surface or where movable ribbons only partially contact the surface of the substrate. Accordingly, FIGS. 1a-b and FIGS. 2a-b are for illustrative purposes only and are not intended to limit the scope of the invention. Further, it is understood that the current invention is not limited to grating light valves and has applications for reducing vibrational oscillations in other micro machine devices with or without reflective surfaces.
FIG. 3 plots an idealized brightness response 107 of a grating light valve to an incident light source with a wavelength xcex when voltage 108 is applied across a selected set ribbons (active ribbons) and the underlying substrate of the grating light valve to alternate between the conditions for constructive and destructive interference. From the discussion above, the brightness will be at a maximum 111 when the ribbons are in the same reflective plane or separated by xcex/2, or a multiple of xcex/2, and the brightness will be at a minimum 111 when the ribbons are separated by xcex/4, or xcex/4 plus (n)xcex/2. Specifically, to operate the grating light valve, a voltage V1 is applied across the active ribbons and the underlying substrate. At this point the active ribbons are in the constructive interference position and the maximum brightness 109 is observed. As the voltage is increased to V2, the active ribbons are moved to a destructive interference position and the minimum brightness 109 is observed. As the voltage is reduced, the active ribbons do return to their constructive interference position when V1 is reached.
The rate (Volt/sec) at which voltage is applied to switch the ribbons of the grating light valve between the conditions for constructive and destructive interference is referred to as the switching rate, and is typically in the range of 4000 to 0.4 Volt/nano seconds. The frequency of at which the grating light valve is switched between the conditions for constructive and destructive interference is referred to as the switching frequency and is typically in the range of 100 KHz to 20 MHz.
Whether a grating light is constructed according to the principles illustrated in FIGS. 1a-b, FIGS. 2a-b, or any other construction including constructions where movable ribbons do not touch the surface of the substrate, there is the tendency for the ribbons to exhibit oscillatory vibrations when they are moved from one position to another.
In applications where the light source used has wavelengths xcex corresponding to the near infrared (ca. 800-4000 nanometers), the distances that the ribbons are displaced to alternate between the constructive and destructive interference positions are between 200 to 1000 nanometers or greater. With greater displacement distances, the ribbons tend to exhibit oscillatory vibrations with greater amplitudes. These oscillatory vibrations reduce the ability of the device to effectively act as a light valve for light sources in the near infrared.
Many print applications utilize light sources that operate with wavelengths corresponding to the near infrared and the visible region of the spectrum. Because the oscillatory vibrations are a significant limitation for grating light valves operating at these wavelengths, grating light valves have had limited use in print applications. Further, the oscillatory vibrations reduce ability to operate under conditions of high switching voltages and high switching frequencies that are desirable for high speed print applications.
What is needed is a grating light valve that exhibits reduced oscillatory vibrations of reflective ribbons. Further, what is needed is a grating light valve that modulates light with minimized oscillatory vibrations at wavelengths in the near infrared for print applications.
According to the present invention the movable ribbons of a grating valves are sealed within a die structure along with a dampening gas environment. The damping gas preferably has a pressure of between 0.5 and 3.0 atmospheres within the die and more preferably between 0.5 and 1.5 atmospheres at 20 degrees Celsius. The dampening gas environment comprises an inert gas or noble group VIII gas including He, Ne, Ar, Kr, Xe, Rn or a mixture thereof. Preferably, the inert gas is 50 more molar percent of the total dampening gas.
The grating light valve comprises a plurality of spatially arranged elongated ribbons with a reflective surface and a substrate element with reflective regions between the ribbons. The grating light valve modulates light by constructive and destructive interference of the reflected light at an incident wavelength xcex when the ribbons, or a portion thereof, are moved by a predetermined distance. The ribbons are moved to switch between a destructive interference position and a constructive interference position by applying the appropriate switching voltages across selected ribbons and the substrate.
The dampening gas environment attenuates oscillatory vibrations or xe2x80x9cringingxe2x80x9d that results from the displacement of the ribbons while alternating between the destructive interference position and the constructive interference position. The current invention is particularly useful for grating light valves operating at wavelengths xcex corresponding to the near infrared (800 to 4000 nm) and where the ribbons are required to move a distance equal to a multiple of xcex/4. The current invention is also particularly useful for grating light valves that operate at high switching rates (4-40 Volts/nano second) and at high switching frequencies (1 kHz and greater).
In accordance with the instant invention, grating light valves are arranged in an array and configured to activate a print medium. The array has a plurality of independently operable grating light valves that are used to activate a pixel or spot in the print medium. The surface of the array is illuminated with a light source, such as a laser source or any other light source suitable for the application at hand. The print medium and the array are moved relative to each other and individual grating light valves or sets of grating light valves are actuated, thereby exposing the medium with the desired image or latent image. The array device and/or the imaging process may also utilize a suitable optical arrangement, including lenses and mirrors positioned between the light source and the array or between the array and the print medium in order to facilitate the imaging process.
The print medium is any print medium that is capable of being activated by the light source used. For example, the print medium is paper that is photo activated with a charge image, which subsequently collects a curable toner to produce an image. Alternatively, the system is configured to activate a developable latent image such as a silver halide-based medium.
The dampening gas environment is preferably sealed within the die structure by providing a metallized gasket on a sealing edge of the die structure. A glass cap is provided with a complementary metallized gasket. The glass cap is placed on the sealing edge of the die and the gaskets are aligned to overlap with a solder material between. The temperature of the die and the glass cap are adjusted and the pressure of the dampening gas environment within the cavity of the die is adjusted. The cap and the die structure are soldered together by virtue of the elevated temperature through the metallized gaskets and the solder material which form a hermetic seal and trap the damping gas environment within the die structure. The total pressure of the damping gas environment within the sealed die structure is preferably in the range of 0.5 to 3.0 atmospheres and most preferably in the range of 0.5 to 1.5 atmospheres.
Referring to FIGS. 9 and 10, a first metallized gasket 550 is formed around a portion of, or a lip portion of, the die structure 501 defining an inner sealing region. A second and complimentary metallized gasket 556 is formed on the lid 558. A grating light valve 500, which is either formed integral with the die 501, or provided separately from the die 501, is positioned within the inner sealing region on the die 501. A solder material 560 is placed between the metallized gaskets 550 and 556 and the temperature is adjusted to a sufficient degree to cause the solder material 560xe2x80x2 to flow and seal the lid 558 to the die 501 through the metallized gaskets 550 and 556. Alternatively, the lid 558 and the die 501 are sealed together using an epoxy material or other adhesive material.
According to a preferred embodiment, the metallized gaskets 556 and 550 are formed from first layers of chromium that are deposited on the lid 558 and the die structure 501 to a thickness of approximately 300 angstroms. A second layers of gold is then deposited on each of the first layer of chromium to a thickness of approximately 10,000 angstroms. It is also preferable, that the solder material 560 is an 80 Au/20 Sn solder that is approximately 50 microns thick. Other details and embodiments are described in U.S. patent application Ser. No. 09/124,710, the content of which is hereby incorporated by reference. The primary function of the metallized gaskets 550 and 556 in the instant invention is to provide a compatible interface between the lid 558 and the solder material 560 and between the die structure 501 and the solder material 560, such that a hermetic seal is achieved.
The step of sealing the ribbon in the dampening gas environment is preferably performed in a isolation chamber where the temperature of the die structure and glass cap are adjusted to approximately 300 Celsius for a sufficient period of time to form the seal as described above. During the formation of the seal or prior to the formation of the seal, the pressure of the dampening gas environment is adjusted to between 1.0 to 6.0 atmospheres, such that the resultant grating light valve device after cooling has a encapsulated dampening gas environment with a pressure that is approximately between 0.5 to 3.0 atmospheres.